[meinhimmel]

WEP can be broken given enough packets since it sends a portion of the key in each packet enabling the user to simply collect packets and break the key. There's a lot an attacker can do such as fake authentication with the server to force it send more of the necessary packets, etc.

WPA on the other hand cannot be broken like this. WPA is broken by capturing the handshake between the router and a user. This handshake can then be brute forced with a dictionary. The reason it's so difficult to crack is because the encryption key is salted with the essid of the router making rainbow tables extremely difficult to pre-compute. However, if the user uses a standard essid that came with the router, then pre-computed rainbow tables can most likely be found for it. They can be computed though while capturing the handshake, and I find it's beneficial to create the rainbow tables if you have a large dictionary.

Take a look at aircrack-ng for a lot more detail. This is just a basic overview of it all.